The following relates to the optical arts, optical delay element arts, and related arts, and to applications of aforesaid such as phased array antennae, optical buffering, and the like.
Phased-array antennas find application in a wide range of systems that rely on the emission and reception of electromagnetic waves. Such systems include surveillance, tracking, astronomy, and geodesy to wireless and satellite communication. Phased-array antennas are made up of a series of independent, small-element antennas that can be programmed to jointly produce a concentrated beam of electromagnetic ray propagating at a certain direction. See, e.g. Anderson et al., “Binary Optical True Time Delay Based on the White Cell: Design and Demonstration, “IEEE Journal of Lightwave Technology,” IEEE Journal of Lightwave Technology, vo. 24 no. 4, pp. 1886-95, April, 2006.
To control the emission and reception directions of the phased-array antenna, each antenna element's emission (or received signal) is phase-shifted or time-delayed by a precise amount to produce a directional propagation (or reception). For narrowband waves, phase-shifting suffices, as it treats a phase shift of 4π as equivalent to 0 radians, which is valid for a narrow range of frequencies. For broadband systems, true-time delay (TTD) is suitably used, in which a time delay that amounts to 4π at one frequency might be 3.5π at another. True-time delays prevent beam squinting, in which different frequencies travel in different directions. Examples of existing optical TTD systems include those based on the White cell. See, e.g. Anderson et al., U.S. Pat. No. 6,266,176 issued Jul. 24, 2001. A known design for a White cell-based TTD system includes a White cell with a micro-electro-mechanical system (MEMS) of mirrors and various optical delay devices operating around the White cell. As an array of focused beams is sent into the White cell, it will collectively reflect within the cell and form pixelated and non-overlapping spot patterns that focus on a successive MEMS element of a MEMS array after every two bounces (for the binary cell). To delay any individual ray within the beam array, for each cycle of operation (every two bounces), the appropriate pixels on the MEMS array can be tilted such that the chosen beams leave the White cell and go into an optical delay element. Over one cycle, the specified optical delay element adds delays to the selected beams with respect to the rays that circulate within the White cell, and it sends the beams back into the system for the next cycle of operation. The delay elements used in conjunction with the White cell should have certain properties. The delay elements should satisfy the imaging constraint—that is, the output beam should be an image of the input beam. The positions and slopes of the rays should have a predictable input/output relationship. Normally, it is desirable to have delay elements that act like mirrors, but this is not a necessity. Additionally, the delay elements should not produce an excessive amount of optical loss.
Various types of delay elements are known, but they have various limitations. Dielectric delay blocks are appropriate for short delays, on the order of picoseconds, but are less practical for longer delays. Delay elements based on lens trains have similar problems: the lens train can become prohibitively long and include many discrete lenses, making alignment difficult. In principle, delays up to perhaps 25 nanoseconds are practically achievable using lens trains. Heretofore, longer delays have typically been achieved using optical fiber delay elements, which also introduce alignment issues and losses.
Disclosed herein are improved delay elements capable of achieving long delays in a compact device with low losses.